The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with medical magnetic resonance imaging systems and be described with particular reference thereto. It is to be appreciated, however, that the present invention may also find application in conjunction with other types of magnetic resonance imaging systems, magnetic resonance spectroscopy systems, and the like.
In magnetic resonance imaging, a substantially uniform main magnetic field is generated within an examination region. The main magnetic field polarizes the nuclear spin system of a subject being imaged within the examination region. Magnetic resonance is excited in dipoles which align with the main magnetic field by transmitting radio frequency excitation signals into the examination region. Specifically, radio frequency pulses transmitted via a radio frequency coil assembly tip the dipoles out of alignment with the main magnetic field and cause a macroscopic magnetic moment vector to precess around an axis parallel to the main magnetic field. The precessing magnetic moment, in turn, generates a corresponding radio frequency magnetic resonance signal as it relaxes and returns to its former state of alignment with the main magnetic field. The radio frequency magnetic resonance signal is received by the radio frequency coil assembly, and from received signals, an image representation is reconstructed for display on a human viewable display.
In certain MRI applications, it is advantageous to perform imaging scans with both a large field of view and a small field of view, high resolution scan. In the past, two separate radio frequency coils were employed, one to image the broad, general large field of view, and a separate radio frequency coil to image the narrower, more specific region of the subject being examined. For example, conventionally, when imaging the head, a quadrature birdcage coil is employed. The birdcage coil gives good uniformity and signal to noise ratio over the entire head. However, when performing imaging techniques such as functional neural imaging, it is advantageous to have very high resolution and signal-to-noise ratios in certain areas of the brain. The birdcage coil is not sufficient for such imaging techniques. Various other volume coils, such as dome top and derivatives of domes have been used. Nevertheless, to get the very high local signal-to-noise ratios, smaller diameter surface radio frequency coils are desirable. That being the case, the traditional way of performing such imaging was to use a whole head radio frequency coil to do the initial imaging. After localizing a specific region of interest within the broader field of view, the whole head radio frequency coil was removed and smaller diameter radio frequency surface coils or coil arrays were installed for imaging the specific region of interest. Likewise, when performing cardiac imaging, a whole-body quadrature birdcage coil was typically employed. This coil provided good uniformity over the entire torso allowing visualization of the heart, coronary arteries and associated vasculature. Alternately, whole-body wraparound coils have been employed. These coils sacrifice uniformity for improved signal-to-noise ratios. However, neither coil is sufficient when more detailed imaging of the arteries and/or other anatomy is desired. Again, detailed imaging of narrow, specific regions of interest was best accomplished by separate radio frequency surface coils. After imaging the broad, general field of view to locate the specific regions of interest, the radio frequency coils were changed to allow for more detailed imaging in the regions of interest.
A second previous method of performing such imaging was to use a large array of many smaller surface coils. When done in this manner, local surface anatomy was imaged well. However, anatomy at the center of the subject being examined had very low signal-to-noise ratios. Therefore, whole scans had extremely poor uniformity.
An inherent drawback of these prior techniques is that the changing of coils often lead to misalignment. Further, often associated with the changing of coils was the removal of the subject from the examination region, which, again, lead to misalignment problems upon the patient's return to the examination region. As well, for interventional MRI applications, it is advantageous to limit patient motion.
The present invention contemplates a new and improved magnetic resonance imaging apparatus which overcomes the above referenced disadvantages and others.